With the advance of information technology, the audio/video files need higher and higher resolution and demand greater and greater storage capacity. A high-quality terminal device should be equipped with a high-performance data transmission system to transmit an enormous amount of data. Thus, ADC (Analog to Digital Converter), which functions as a conversion interface, demands higher and higher specification, some of which may be far beyond the range that the testing stimulus signal generators can operate. Hence, the high-resolution ADC is usually performed verification by lowering the resolution thereof during testing. Consequently, the test results are usually unpractical.
A US Publication No. 20090040199 entitled an “Apparatus for Testing Driving Circuit for Display” discloses an analog-to-digital converter having a ramp generator. The ramp generator generates a linear triangular wave or a ramp wave (the so-called testing stimulus signal) for testing the analog-to-digital converter. The fundamental problems of a ramp generator include whether the linearity of signals can be used for testing the circuit under test having higher and higher resolution, whether it is expensive, whether the test result thereof is as accurate as expected when considering the non-ideality of the fabrication process, whether it can overcome the factors of environmental interference, probe pointing, loads, etc., and whether it is practical to generate testing stimulus signals externally to input to a chip in case of SOC (System-on-a-Chip). A digital-to-analog converter can provide testing stimulus signals. However, a high-resolution digital-to-analog converter built in a chip not only is expensive but also increases the complexity of design and integration of the chip.
Another typical method for generating testing stimulus signals is to connect a constant current source to a capacitor. Refer to FIG. 1. Via a constant current source 1 and a capacitor 2, the output current can be converted into the voltage drop of the capacitor 2, which is a testing stimulus signal desired. The constant current source 1 is provided by incorporating a current mirror with great output impedance. Such a method is instinctive. However, the method can only apply to a chip where a constant current source 1 and a capacitor 2 exist simultaneously. Refer to FIG. 2. In practice, the constant current source 1 and the capacitor 2 are non-ideal and have parasitic effects which causes the stray charging curve 3 pretty different from the ideal charging curve 4.